This invention relates to magnetic field measuring devices and methods and more particularly, to compasses and systems that reference the Earth's magnetic field vector.
Magnetic field measuring devices are used to determine the Earth's magnetic field vector at a particular geographic location. The accurate characterization of the Earth's magnetic field is dependent upon the measuring device's ability to compensate for the magnetically susceptible material and magnetic sources that may be contained within the device itself or the vehicle supporting the device. Thus, the ability of the measuring device to report an accurate representation of the undistorted field is dependent upon the method utilized to compensate for local distributions of magnetically susceptible material and local magnetic sources within the device or vehicle supporting the device.
The primary shortcoming of the prior art is the inability to accurately compensate for the change in the local magnetic field due to local distributions of magnetically susceptible material in the vehicle supporting the magnetic field measuring device. The magnitude and direction of the Earth's magnetic field varies with geographic location. Of particular importance is the dip angle, the angle between the field vector and the horizontal plane. Distortions due to magnetically susceptible material in the vehicle depend upon field strength and direction relative to the vehicle reference frame. Thus distortion of the Earth's magnetic field due to local distributions of magnetically susceptible material in the supporting vehicle varies with respect to geographic location and vehicle orientation. The prior art uses calibration methods to compensate a magnetic field measuring device for the distortions encountered at a particular geographical location over a limited range of vehicle orientations. As a result, when the vehicle assumes an orientation not tried in the calibration procedure, or when the measuring device is transported by the vehicle to a geographic location which differs from the location where the instrument was calibrated, new and unknown compensation factors are required to obtain the undistorted field. Thus, the accuracy of the device in characterizing the magnetic field is unpredictable at untried vehicle orientations and at locations outside the immediate calibration area.
The present invention overcomes the aforementioned short comings by measuring the total field vector, by calibrating over a sufficient range of vehicle orientations, and by processing data through an accurate mathematical model of the physical process involved.
It is an object of the present invention to provide an improved method and apparatus for measuring the Earth's magnetic field from within a vehicle or other support system for the measuring device. Presently the most practical application is a compass for aircraft, ships, boats, land vehicles and various instruments.
Another object is to provide an apparatus and method for compensating for inaccuracies in measuring the Earth's magnetic field due to materials within the supporting vehicle.
Still another object is to provide a measurement of the Earth's magnetic field which is independent of changes in the localized magnetic field due to local distributions of magnetically susceptible material and independent magnetic sources.